313 



that here, also, as in the omission of characters in the earlier stages of 

 ontogeny, the heritage is incomplete. 



Of the complications of inheritance that arise from larval adapta- 

 tions, intra-uterine adaptations, and special adaptations arising in later 

 life, I shall not speak. All of these have been repeatedly discussed (see 

 for example Smith 57), and are well understood. Against all of these 

 the paleobiologist must be on his guard. All of these factors tend to make 

 the parallelism between ontogeny and phylogeny inexact, as long ago 

 pointed out by Cope (15). Yet in spite of the operation of these factors, 

 the cases in which there is clear evidence of recapitulation are so numer- 

 ous, and so well known to the paleobiologist, that were it not for the 

 continually reiterated statements of certain embryologists that there is 

 no such thing as recapitulation, I should hesitate to again point them 

 out. I shall now take up the evidence according to the groups of or- 

 ganisms in which it has been ascertained ; and I once more remind the 

 reader that most of this evidence applies to the epembryonic and not to 

 the embryonic stages. 



II. 



Cephalopoda. — The only existing representative of the great group 

 of Tetrabranchiata, the class to which nearly all of the fossil cephalopods 

 belong, is the Nautilus. The genus Nautilus is a striking example of the 

 persistence of a primitive type. It belongs to the more primitive branch 

 of the tetrabranchs, from' which, according to all the evidence, the marvel- 

 ously complex ammonities, on the one hand, and the modern naked cepha- 

 lopods are descended. Nautilus is the only tetrabranch of Avhich the 

 entire ontogeny, including the embryonic stages, is known. 



This lack, however, in the case of the fossil genera is not as serious 

 as might be supposed, for the reason that even in these ancient forms all 

 of the growth stages from the latest embryonic (phylembryonic) stage to 

 the adult are preserved in every complete individual shell. An inspection 

 of the Nautilus shell makes this at once apparent, for the earlier stages 

 of the shell are surrounded and protected by the later, and no part of 

 the shell is lost or resorbed. In the straight and loosely coiled shells 

 only, such for example as Orthoceras, Gyrtoceras, etc., is the case different; 

 and even here, barring injury, or the dehiscence of the earlier chambers, 

 every post-embryonic stage is preserved. From a study, therefore, of a 

 single shell, we are able to make out perfectly all of the epembryonic de- 



